Who is Participating?

Yes, ADMT can also perform intra-forest migrations. I haven't done it and I couldn't find a source that described your environment using ADMT 3.0. However, the principle is described in chapter 11 of the Domain Migration Cookbook [1].

The first part of scenario 1 and the whole of scenario 2 are both candidates for the Active Directory Migration Tool [1]. ADMT terms these both as Inter-forest migrations [4]. Inter-forest migrations perform a copy of the objects and so make it easy to test the migration prior to the live date.

The second part of scenario 1 is termed a Intra-forest migration. These can be a little trickier and so require more planning.